1. Field of the invention
The present invention relates to a two-dimensional image reading device and, more particular, to an arrangement of a solid state imaging unit which can detect, without employing any mechanical or optical scanning arrangement, an image on a document which is placed directly on the imaging unit.
2. Description of the Prior Art
An image reading device is used in an apparatus, such as a facsimile machine or an intelligent copier. In the prior art image reading device, the document is read in a so-called slit scan method wherein the document is illuminated in a slit which is optically or mechanically scanned across the entire document. The slit image, which is usually reduced into a smaller size by a suitable optical lens system, is detected and read by the solid state imaging unit which is arranged in a one-dimensional layout, such that a plurality of imaging elements, such as CCDs or MOS type, are colinearly aligned.
A typical solid state imaging unit of prior art is about 30 millimeters long and, therefore, it is necessary to provide a relatively long optical path between the document where the slit image is produced and the imaging unit on which the slit image is reproduced. Accordingly, it has been very difficult to provide an image reading device of a compact size. Furthermore, in such a prior art image reading device, the adjustment in the optical system, such as a focusing adjustment, has been very difficult requiring a high skill. In addition, there are many other problems, such as a problem of insufficient light intensity in the peripheral portions and a problem of resolvability.
In order to overcome the problems described above, a so-called contact type of image reading device has been proposed. According to this device, a one-dimensional solid state image sensor is employed which can directly detect and read a slit image, without any reduction of the image, by a fiber lens array which extends between the document where the slit image is produced and the imaging unit. According to the contact type of image reading device, however, it is necessary to provide a photoelectric converting device of a relatively large size, requiring a formation of a photoconductive layer uniformly over a relatively wide area. At present, in order to form a one-dimensional solid state imaging unit of the contact type, it is proposed to use a CdS-CdSe layer, Se-As-Te amorphous layer or amorphous Si layer. In any case, to employ these layers to an image reading device, it is necessary to provide an arrangement to scan the document or to scan the optical lens system together with the imaging unit across the document, resulting in a bulky and sophisticated arrangement.